Question

Discuss the process of translation in detail.

Answer 1

Translation:

Translation is the process of synthesis of polypeptide chains from mRNA. It occurs in the cytoplasm.

Steps involved in translation:

Three steps are involved in translation as follows:

● Activation of amino acids.

● Charging of tRNA.

● Formation of polypeptide chain or protein synthesis proper.


Activation of amino acids:

Amino acids are activated in the presence of ATP and linked to their tRNA.

Amino acid

ATP $\downarrow$ Aminoacyl -tRNA synthetase
Enzyme -AMP - Amino acid


Charging of tRNA:

The amino acid bound with ATP is transferred to the corresponding tRNA with liberation of AMP. This process is called aminoacylation of tRNA or charging of tRNA.


Protein synthesis proper:

Protein synthesis or formation of polypeptide chains occurs in the ribosomes. mRNA is read in 5’$\to$ 3’ and protein synthesis proceeds from N - terminal to C - terminal.

It involves three steps as follows:

● Initiation

● Elongation

● Termination


Initiation of translation:

● Initiation of translation occurs by the attachment of small ribosomal subunit to the region of start codon on mRNA (30s in prokaryotes and 40s in eukaryotes).

● After the small subunit comes and joins the mRNA, the initiator tRNA with the amino acid methionine comes and binds to the AUG codon.

● This is known as the initiation complex.

● In prokaryotes, 30S - mRNA + tRNA (fMet) $\to$ 30S - mRNA - tRNA (fMet).

● In eukaryotes, 40S - mRNA + tRNA (Met) $\to$ 40S - mRNA - tRNA (Met).

● After the formation of the initiation complex, the large subunit joins with the small subunit followed by further process of translation.


Elongation of translation:

● After formation of ribosome -mRNA - tRNA complex, A - site (aminoacyl acceptor site) is established near P - site.

● In A - site, a new aminoacyl tRNA forms codon - anticodon bonding.

● The peptide bond is formed in between the COOH group of methionine and amino group of second amino acid in tRNA, catalyzed by the enzyme peptidyl transferase.

● The ribosome moves from codon to codon in mRNA and the amino acids are added one by one.

● Elongation of translation is accompanied by the movement of ribosomes along the mRNA towards its 3’ end. This is called translocation and requires energy by the hydrolysis of GTP.


Termination of translation:

● Termination is signalled by stop codons such as UAA, UAG and UGA.

● The release factor is associated with the stop codons, and it terminates transcription and releases the mRNA.

Pictorial representation of translation: